This invention relates to a rubber composition for tire treads which is capable of providing a tire satisfying both of the requirement for reducing the rolling resistance and that for improving the wet braking property of the tire.
In recent years, in view of an increasingly made public demand for saving resources and energies, there have been made studies toward development of automobiles which call for no heavy gasoline consumption and it is sought for to expedite studies devoted to the so-called fuel-saving tires capable of alleviating the rolling loss, besides studies on improvement of engines.
In this connection, it is widely known that the consumption of gasoline by an automobile is cut by decreasing the resistance to the rolling of its tires. The quality, namely kind and physical properties, of the tread rubber is counted among major factors which determine the rolling resistance of tires. Desired decrease of the rolling resistance can be attained by using a material of a low glass transition point, or a material of a low hysteresis loss. For example, it is known that the styrene-butadiene copolymer rubber (SBR) which is widely used as tread rubber is substituted with a low-styrene SBR or a blend of SBR with natural rubber (NR) or polybutadiene rubber (BR) to reduce the rolling resistance. However, these substitutes extremely degrade the traveling stability such as the wet skidding property and wet braking property. In this respect, it is known to be extremely difficult to attain a reduction of the rolling resistance and yet attain an improvement in the traveling stability on wet road surfaces.
It has been introduced to the art by, for example, Proceedings of The Royal Society A 274,21 (1963) that for the purpose of improving the coefficient of friction on wet road surfaces, it is recommended to use, as tread rubber, a material which has a large value of the hysteresis loss and can dissipate the deforming energy exerted to rubber. This is because the surface of tire tread in contact with the road surface is deformed with rapid bobs by fine rises and falls on the road surface and, as a result thereof, the frictional force increases in proportion as the dissipation of energy by the mechanical loss increases. However, when the tire tread is made of a material having a higher hysteresis loss, it generates a larger amount of heat during the rolling of tire and does not exhibit a desirable resistance to the rolling.